Researcher sees wild rainfall fluctuations ahead

UCR associate professor Robert Allen of Riverside has published a study with predictions suggesting California’s climate may actually become 12 percent wetter between now and the end of the century.
Terry Pierson — The Press-Enterprise/SCNG

In the middle of a heat wave it’s easy to yearn for rain, and at least one researcher says the rain will come — lots of it.

UC Riverside climate researcher Robert Allen says California should get ready for more rain.

Unlike other recent work in the field, Allen has just published a study that says rainfall in the state will increase in the coming years if the planet continues to warm at its current pace.

Allen, 42, is a professor of earth sciences and has been studying climate issues for two decades. His study projects that by 2100, precipitation in California will rise 12 percent.

Southern California rainfall won’t change much. In fact, the models predict it will drop slightly, but the increases in the rest of the state may be more than 15 percent higher than current averages. In the winter months, those percentages go up nearly 40 percent.

So, does that mean residents of the state can forget about conserving water and go ahead and dig the hole for that new swimming pool? Not exactly.

Allen says the past few years, where a severe drought was followed by near-record rain and snow, is likely the type of climate swing we can expect as we head through the century.

“The average (rainfall) is the average,” Allen said, “but the standard deviation is quite high. The standard deviation will increase.

“My study shows an increase in mean precipitation,” he added, “but you could also see an increase in drought. When it does rain, it rains a lot.”

The study appears online in the journal Nature Communications. Allen worked with graduate student Rainer Luptowitz on the NASA-funded project.

Past studies have typically predicted a decrease in rainfall in California as global temperatures rise — although many studies have indicated such factors are difficult to assess.

Allen said it’s generally accepted that tropical areas will become wetter, as will regions at higher latitudes. Subtropical climates will become drier. California includes both subtropical and higher latitude regions.

“Precipitation is a much more difficult parameter (than temperature) to project into the future,” he said.

California is even more difficult to model, because it overlaps those two climate regions. Allen has looked at studies in the past, but was particularly intrigued by a recent study by some UCLA scientists that predicted warming ocean temperatures would cause the Pacific jet stream to shift to the southeast. If so, it could potentially bring more moisture to California.

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Allen said he had about 40 climate models to work with in looking at the phenomenon. He screened the models by their ability to accurately model the known climate of California since satellite weather data began being used in 1979.

The most recent models, he said, proved to be the most accurate. The 14 models he focused on predicted a much wetter climate going forward.

The models said precipitation would increase due to what Allen called an “El Niño-like atmospheric response.” He said that the condition was independent of an actual El Nino, which occurs when a band of water in the central and east-central Pacific near the equator warms. Since surface water temperatures are expected to warm worldwide, Allen said that could create a sort of constant El Niño condition.

“There’s this El Niño-like response in a warmer world that acts to drive this increase in precipitation,” he said.

But, just as there are El Niño years, such as 2015, when the phenomenon results in below normal rainfall for California, Allen said nothing is certain.

“Even if precipitation increases, you can still have drying,” he said. “We know that climate change is going to be associated with enhanced evaporative demand.”

Warmer temperatures may mean farmers will need to use greater amounts of water as more is lost to the atmosphere.

He also said that rainfall is a single component in climate modeling. Other components can be just as important.

“This is just one study,” he cautioned.”I don’t think one study should ever be the definitive source or should lead to changes in public policy or political decisions.”

He expects further work on the issue will be conducted using a set of new climate models that are due to be out next year.